Automatic track insertion and display

ABSTRACT

Digital values of designated track angles for various portions of a preplanned flight are presented by a storage unit for different segments of the flight plan. These values are converted to quadrature square waves of a phase related to the designated track angle, which are used to synchronously demodulate sinusoidal signals of X and Y coordinates from the aircraft to a way point relating to the flight plan segment, the result being combined to provide along-track distance to way point and cross track error. The square wave signals may be used to drive a conventional differential resolver so as to automatically adjust the resolver to the designated track angle to provide an indication thereof; alternatively, a digital display of the stored value may be used.

United States Patent [1 1 Brock et al.

[ 1 Dec. 11, 1973 [54] AUTOMATIC TRACK INSERTION AND 3,534,399 10/1970Hirsch 235/150.27 DISPLAY, 1 3,464,016 8/1969 Kerwin et a1. 235/186 X w3,691,356 9/1972 Miller 235/1S0.22 [75] Inventors: Larry D. Brock,Collmsville; John Games Granby both of Conn Primary Examiner-Ma1colm A.Morrison [73] Assignee: United Aircraft Corporation, East AssistantExaminer-Jerry Smith Hartford, Conn. Attorney-Melvin Pearson Williams[22] Filed: July 3, 1972 7 B T A T [21] Appl. No.:'268,252 t [5 1 S R CDigital values of designated track angles for various 1 portions of apreplanned flight are presented by a stor- [52] 0.8. CI 23S/l50.27,235/150.26, 235/186, age unit for different segments of the flight plan343/112 D These values are converted to quadrature square [51] Till. C1.G06g 7/22, G06g 7/78 waves of a phase related to the designated trackangle, [58] Field of Search 235/150.2, 150.26, which are used tosynchronously demodulate Sinusoi 235/150'27 186; 343/106 1071 H2 dalsignals of X and Y coordinates from the aircraft to 343/112 D; 244/77 Ba way point relating to the flight plan segment, the result beingcombined to provide along-track distance to [56] Reierences cued waypoint and cross track error. The square wave sig- UNITED STATES PAT NTSnals may be used to drive a conventional differential 3,659,291 4 1972Anthony 343/106 R x s l r s as t automatically adjust the resolver to3,671,728 6/1972 Day et a1. 235/186 X the designated track angle toprovide an indication 3,406,280 10/1968 Vago 235/150.27 thereof;alternatively, a digital display of the stored Vago value may be used3,621,212 11/1971 Hobbs eta1.... 235/150.27 3,581,073 5/1971 Visher235/150.26 4 Claims, 6 Drawing Figures Av X J//\/ W7 y c Z o TX Z? 7 xJflJfl-KI/A/f ln-j0(M/f g f w 14a /J0 g gz F/df 0/914? L Jm/c Z7 1 6FfiflM H6; crufiffl f f flj 07; 3% ZZ w- YJ/A/ W7 JYA/C (0/ 74 A? iZ v 602.7 @a W a r l:rJfl(M/7' i VJT/A/ X flu t/ a LIV/V6 flf/Wflfi X50;

Z? rum/v7 dwa) PAIENIEBBEB 1 1 ms SHEET 10F 3 away d AUTOMATIC TRACKINSERTION AND DISPLAY BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to aircraft navigation computers, andmore particularly to an aircraft track computer.

2. Description of the Prior Art Navigation of aircraft in response toindications (or command signals to an automatic pilot) from navigationequipment is well known. One of the parameters which such equipment hasbeen known to provide is the cross track error; another is a to-fromindication to determine whether the aircraft is flying toward the waypoint or away from it. Typically, a way point is selected in response tomanual controls which provide signals indicative of the coordinates ofthe way point with respect to the location of a known VOR/DME groundstation. These signals are combined with the coordinates to the groundstation as provided by the VOR/DME receiver and coordinate conversionequipment on board the aircraft so as to provide coordinates from theaircraft to the selected way point. These signals are then used to feeda differential resolver, the rotor of which is adjusted to the selectedtrack angle by the pilot in response to charts or tables indicative ofthe current segment of a preplanned flight.

It has been found that the setting of the resolver rotor to the desiredtrack angle is subject to relatively high error; for instance,,at ahundred miles distance from a way point, an error of 1 it" in the manualsetting of the track angle can result in a 2.6 mile cross track error.Additionally, the manual setting-in of the coordinates and track anglecomprises manual workload for the pilot.

SUMMARY OF THE INVENTION A primary object of the present invention is toprovide more accurate track angle designation in an aircraft navigationtrack computer.

Another object of the present invention is to provide a track computerwhich reduces pilot workload.

According to a first aspect of the present invention, a designated trackangle relating to a selected way point is represented by digitalindications which are combined, in an electronic computational means,for signals representative of said selected way point to accuratelycompute cross track error; further the electronic computational meansprovides a to-from indication from the sense of the along-track distanceof the aircraft to the selected way point.

According further to this aspect of the invention, a designated trackangle is represented by signals, the phase of which are proportional tothe designated track angle, and these signals are combined with signalsrepresentative of the coordinates of the aircraft with re spect to thecorresponding selected way point so as to provide signals relating tocross track errors and for op erating the to-from flag. Morespecifically, the invention provides quadrature square wave signals thephase of which is representative of the designated track angle, whichsignals are utilized to synchronously demodulate sinusoidal signalsrepresentative of the X and Y coordinates of the aircraft with respectto the way point.

In accordance with another aspect of the present invention, signalsrepresenting designated track angles are automatically provided, foreach segment of a preplanned, prestored flight by a storage means.

Since the present invention avoids the need of reliance upon adifferential resolver to provide signals for determining cross trackerror and to'from indications, the differential resolver may beeliminated entirely if desired; the storage unit instead providing adigital display of the designated track angle. On the other hand, thepresent invention utilizes the signals representative of the designatedtrack angle to drive a differential resolver so as to automatically setit to the designated track angle thereby to provide an indication forthe pilot.

The invention achieves great accuracy since it does not rely on humanintery ention in producing the signals utilized for the computation ofcross track error, or of the to-from indication, but rather provides anaccurate preflight insertion of desired track angles. It further reducespilot workload and permits smoother transitions from one segment of apreplanned flight to another. The invention uses electroniccomputational means, thus avoiding reliance on electro-mechanicalresolvers. The invention is easily implemented with low cost andreliable components readily available in the art. The invention may beimplemented, in its broadest sense, with digital computational means, ifdesired.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof preferred embodiments thereof, as illustrated in the accompanyingdraw- BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram illustrativeof the trigonometric relations between way point coordinates and trackan- 3 FIG. 2 is a schematic block diagram of demodulating and combiningcircuitry suitable for use in the present invention;

FIG. 3 is a schematic diagram of known apparatus for providing signalsrepresenting aircraft X, Y coordinates to a selected way point;

FIG. 4 is a simplified schematic block diagram of apparatus responsiveto stored, designated track angle signals to produce square wave signalshaving a phase relationship to the designated track angle for use in anembodiment of the present invention;

FIG. 5 is a circuit for generating signals for driving the resolver toprovide an indication of designated track angle; and

FIG. 6 is a schematic diagram of circuitry for driving a resolver toprovide a designated track angle indication in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 anaircraft is assumed to be at a position 10 having X and Y coordinates ina cartesian coordinate system, the origin of which is located at aselected way point 12. Associated with the way point 12 for a givensegment of a preplanned flight is a designated track angle d). A crosstrack error results when the actual track angle of the aircraft isdifferent than the designated track angle. The cross track error(C.T.E.) can be computed from trigonometric relationships as C.T.E. Xcos Y sin d) Similarly, the along-track distance (A.T.D.) can becomputed as A.T.D. X sin (1) Y cos d:

This is achieved, in one embodiment of the invention, as illustrated inFIG. 2. Therein, a plurality of synchronous demodulators 14-17 utilizesquare wave signals, the phase of which is a function of the designatedtrack angle, to synchronously demodulate, in various combinations,sinusoidal signals representing the X and Y coordinates of the aircraftwith reference to the selected way point. Specifically, the synchronousdemodulator 14 has at its reference phase input a 460 Hz square wavesignal on a line 18 which is delayed in phase by d) fron sin wt, tosynchronously demodulate a signal on a line 20 equal to X sin wt. Thesynchronous demodulator 15 utilizes at its reference phase input asignal on a line 22 delayed in phase by d: from cos wt, to synchronouslydemodulate the signal on the line 20. The synchronous demodulator 16utilizes at its reference phase input a signal on the line 22 tosynchronously demodulate a signal on a line 24 equal inverted to Y sinwt, and the synchronous demodulator l7 utilizes the signal on the line18 to synchronously demodulate the signal on the line 24. The output ofthe synchronous demodulator 14 on the line 26 is equal to X cos (1:; theoutput of the synchronous demodulator 15 on the line 27 is equal to Xsin (1); the output of the synchronous demodulator 16 on the line 28 isequal to Y sin d and the output of the synchronous demodulator 17 on theline 29 is equal to Y cos 4:. A signal equal to X cos (1; Y sin 11) isdeveloped on a line 30 by an amplifier 32 having a pair of input summingresistors 33, 34 to which the signals on the lines 26 and 28 areapplied. A signal equal to X sin (1; Y cos d) is provided on a line 36by an amplifier 38 having input summing resistors 40, 41 to whichsignals on the lines 27 and 29 are applied.

By comparison with FIG. 1, it is seen that the signal an the line 36comprises the cross track error and the signal on the line 30 comprisesthe along-track distance.

Signals on the lines 30, 36 are passed through respective low passfilters 42, 44 which may be on the order of 2 to 10 Hz; these filterseliminate the reference frequency (wt) ripple in the output to form asubstantially DC signal proportional to cross track error and alongtrackdistance, respectively. The output of the filter 42 may be applied to anisolated driver 46, if desired for driving an isolated cross track errorindicator of the type known in the art with a cross track signal on aline 47, and the output of the filter 44 may be applied to a polaritysensing circuit 48, which in turn may comprise either a low thresholdhigh gain amplifier or a comparator referenced to ground, so as toprovide the to-from signal on a line 50, the polarity of which is alwaysrelated to the polarity of the along-track distance to or from the waypoint as the case may be.

The circuit of FIG. 2 is illustrative of a primary aspect of the presentinvention; that is, providing highly accurate cross track error andto-from indications electronically, eliminating reliance upon the manualsetting of a differential resolver. The source of the signals may varyin any given implementation of the present invention. illustrates FIG. 3ILLUSTRATES a source of signals representative of the X and Ycoordinates of the aircraft with reference to the way point, of a typethat is known in the art. Therein, a VOR/DME airborne receiver andcoordinate converter 52 provides x and y signals to respectivedifferencing circuits 53, 54 which also receives x and y signals from amanually adjusted way point selecting apparatus 56. The output of thedifferencing circuits on respective lines 58, 59 comprise signalsproportional to the X and Y coordinates, respectively, of the aircraftwith respect to the selected way point. In some embodiments, thesesignals may be AC signals and may therefore be applied directly to FIG.2. In other embodiments, these signals may comprise DC signals and wouldtherefore require modulation before utilization in FIG. 2. For thispurpose, a pair of modulators 60, 61 each utilize a 460 Hz referencesignal on a line 62 at the respective reference phase inputs thereto toprovide sinusoidal signals on the lines 20, 24 for application to thecircuit of FIG. 2. In this embodiment, the 460 Hz reference is a squarewave developed in FIG. 4; the modulators 60, 61 may comprise choppersfeeding 460 Hz band pass filters.

In accordance with another aspect of the invention, the square wavesignals having phases relating to the designated track angle on thelines 18 and 22 in FIG. 2 are derived as illustrated in FIG. 4, whereina storage device 64 has stored therein, in BCD code, values of aplurality of track angles, each designated for a particular segment of apreplanned flight, each corresponding to a predesignated way pointrelating to the same flight segment. Through any suitable manual inputmeans 65 the track select mechanism 66 associated with the storagedevice will cause the storage device 64 to provide on a trunk of 14output lines 68 signals representing the designated track angle value.The storage device 64 may simply comprise a random access memory, havinga suitable switch to provide inputs thereto which automatically decodeto the prestored output value. Other addressable storage devices knownin the art may be used as desired. The designated track values on thetrunk of lines 68 may be presented to a digital display device 70 whichwill display the value in degrees and tenths of a degree from 000.0 to360.0. Since 360.0 represents tenths, units, and tens, and only valuesof zero to three in the hundreds column, the BCD code requires four bitseach for the three lowest orders but only two bits for the highestorder, since values of zero to three can be represented with two binarybits. This limits the required word size to fourteen bits. The signalson the trunk of lines 68 are also applied to a 14 bit BCD comparecircuit 72 which also receives signals on another trunk of 14 lines 74from a 14 bit BCD counter 76 which is advanced by signals from asuitable clock 78. If the clock 78 operates at 1.656 MHz, the 360021count-down by the counter 76 provides the 460 Hz reference on the line62. The stages of the counter 76 should be arranged to count from 000.0up to 360.0 and return to 000.0 in a closed-ring fashion; the 000.0count equals zero phase in the period of the 460 Hz reference. It iswithin this cycle that the phase of the square wave signals aregenerated to relate to the designated track angle. The compare circuit72 will issue a signal at the time that the counter 76 presents signalson the lines 74 having the same value as the signals on the lines 68.For a small angle, this will occur at a very low count, and thereforeearly in the period; for larger angles, a larger count will be-requiredso that the compare circuit will provide a signal on the line 80 laterin the period. Thus the signal on the line 80 may vary in phase from 0to 360 with respect to the start of the period (as is defined by the000.0 setting of the counter 76). The clock 78 also drives a 13 bit BCDcounter 82 which is adjusted to count from 000.0 to 180.0, and return to000.0 and continue counting in a closed-ring fashion. However thecounter 82 is provided with a forced reset input 84 which responds to asignal on the line 80 to reset the counter 82 to an all zeros condition.The output of the counter 82 is decoded by suitable logic 86 so as toprovide a first output signal when the count has reached 900 and againto provide a signal on a line 90 when the count reaches I800. The signalon the line 88 therefore defines quarter periods and the signal on theline 90 defines half periods of the time be tween reset pulses. Thuseach period starts with a compare signal on the line. 80. The quarterperiod, comparable to 90 in phase lag, is defined by one signal on theline 88 the half period is defined by the signal on the line 90, and thethree quarter period is also defined by the signal on the line 88. Thesesignals are utilized to generate the two square wave signals on thelines 18 and 22 by controlling respective bistable circuits 92, 94.Specifically, a square wave signal which is in phase with the 460 Hzcarrier (sin wt) except for a phase lag equal to the designated trackangle setting is generated on a line 18 by the bistable 92 since it isset at the time of a comparison signal on the line 80 and is reset l800counts or one half cycle later by the signal on the line 90. On theother hand, the bistable 94 is set and reset in response to the quartercycle signals on the line 88. To ensure that this provides a signalwhich is basically in phase with cos wt, except for the lag therefrom ofthe designated track angle da, an AND circuit 95 will operate inresponse to the signal on the line 88 only when another bistable 96 isreset. This is reset by the compare signal on the line 80. Thus, thebistable 94 is reset 90 after the bistable 92 is set. The /4 cyclesignal on line 88 is delayed (97) and sets the bistable 96; the next V4cycle signal (at 270) will cause an AND circuit 98 to set the bistable94. This assures that a phase relationship to the cosine, rather than tothe inverse of the cosine, will result.

Although one aspect of the present invention relates to the provision ofstored track angles and generation of signals for controlling theresolution of X and Y coordinates (of the aircraft with respect to arelated way point) into cross track and along-track signals, it shouldbe understood that generation of suitable signals for use in theembodiment of FIG. 2 may be made in other ways, if so desired, whileutilizing the primary aspect of the present invention as illustrated inFIG. 2.

As described hereinbefore, if desired, the digital display 70 (FIG. 4)may be used as the sole indication to the pilot of his designated trackheading. However, since numerous aircraft already have a differentialresolver which pilots are accustomed to viewing for an indication ofdesignated track angle, the present invention also provides forutilizing the signals on the lines 18 and 22 for driving thedifferential resolver simply to provide an indication; it should beunderstood that the resolver is not used to provide signals forresolving cross track error and along-track distance as describedhereinbefore. Additionally, as is seen in a commonlyowned copendingapplication of J. E. Games, Ser. No.

268,253 entitled TRUE DISTANCE TO WAY POINT AND OFFSET COMPUTER, filedon even date herewith, the differential resolver is not necessary toprovide distance to way point. Thus the differential resolver, inaccordance with this invention and the aforementioned copendinginvention, can be eliminated entirely or used simply as an indicator.

FIGS. 5 and 6 illustrate how the signals on the lines 18 and 22 may beutilized for this purpose. Specifically, the signals on the lines 18 and22 are applied to the phase reference inputs of a pair of respectivesychro nous demodulators 106, 108 to synchronously demodulate the 460 Hzreference signal on the line 62. This provides signals on correspondingoutput lines 110, 112 which are equal to cos (6 and sin d) respectively.These are passed through suitable low pass filters 114 and applied tomodulators 116, 118 to develop signals on respective lines 120, 122which are proportional to a reference voltage times sin d: sin wt andthe reference voltage times cos (1) sin wt, respectively. These may bepassed through bandpass filters 124 to eliminate unwanted harmonics andthereafter applied on signal lines 126 to a 60 phase rotator 128illustrated in FIG. 6. This provides suitably phased carrier signalswhich are phase modulated in a manner relating to the designated trackangle d), which signals may be passed to a pair of drivers 130 fordriving corresponding windings 132 of a differential resolver 134. Oneoutput winding 136 of the differential resolver may be used to derive anulling output, which can be applied to the signal input of asynchronous demodulator 138, the phase reference of which is driven bythe 460 Hz reference signal on the line 62. The output, afteramplifiction in an amplifier 140, may drive a motor 142 which, through agear train 144, will rotate the rotor 146 of the differential resolver134 to a null, whereby the rotor 146 will point to the designated trackangle and provide an indication thereof to the pilot. If desired, aswitch 148 may be transferred to the position opposite to that shown inFIG. 6 so as to simply short out the winding 136; this may be done inthe case where a differential resolver has enough inherent torque so asto null itself in response to eddy currents, making it possible toeliminate the use of the apparatus 138-144; in fact, in such a case, theswitch 148 may be eliminated simply by grounding the related side of thewinding 136.

Thus the aspects of the invention include generation of along trackdistance and cross track error electronically, without the use of aresolver, providing automatic designated track angle signals in responseto storage, and providing means for driving an indicator to indicate theautomatically designated track angle.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described typical embodiments of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A navigation computer comprising:

means providing a first carrier signal X sin wt and a second carriersignal Y sin wt, the magnitudes of which are representative of X and Ycoordinates of an aircraft with respect to a selected way point;

storage means presenting a selected one of a plurality of designatedtrack angles in the form of a multibit group of digital signals;

a counter, the output of which is representative of track angles, saidcounter being advanced from a zero count to a count relating to maximumtrack angle during each cyclic period;

means comparing the output of said storage device with the output ofsaid counter and for generating a signal when the outputs thereof areequal, and means responsive to said compare output signal for generatinga first square wave signal at the same frequency as said carrier signalbut delayed in phase from sin wt by a designated track anglecorresponding to said selected way point, and a second square wavesignal at the same frequency as said carrier signals but delayed fromcos wt by said angle 11:;

means for combining said carrier signals with said first square wavesignal to generate cross track error as X cos d) Y sin 4); and

means for combining said carrier signals with said second square wavesignal to generate along-track distance as X sin (b Y cos d).

2. An aircraft navigation computer according to claim 1 wherein saidsquare wave signal generating means includes further means responsive tosaid compare output signal for generating signals delayed one quarterperiod, one half period and three quarter periods therefrom forcontrolling said square wave signal generating means.

3. A navigation computer comprising:

means providing a pair of carrier signals the magnitudes of which arerepresentative of orthogonal coordinates of an aircraft with respect toa selected way point;

storage means presenting a selected one of a plurality 8 of designatedtrack angles in the form of a multibit group of digital signals;

a counter, the output of which is representative of track angles, saidcounter being advanced from a zero count to a count relating to maximumtrack angle during each cyclic period;

means comparing the output of said storage device with the output ofsaid counter and for generating a signal when the outputs thereof areequal; 1

means responsive to said compare output signal for providing square wavesignals at the same frequency as said carrier signals, one of saidsquare wave signals delayed in phase from the sine of said carrier by adesignated track angle corresponding to said selected way point, and theother of said square wave signals delayed from the cosine of saidcarrier signals by said designated track angle;

four synchronous demodulators, a first pair thereof synchronouslydemodulating respective ones of said coordinate carrier signals inresponse to one of said square wave signals, the other pair thereofsynchronously demodulating respective ones of said coordinate carriersignals in response to the other of said square wave signals; and

means for summing the output of each synchronous demodulator with theoutput of the other synchronous demodulator of the same pair, theoutputs of said summing means comprising cross track error and alongtrack distance to the way point.

4. An aircraft navigation computer according to claim 3 wherein saidsquare wave signal generating means includes further means responsive tosaid compare output signal-for generating signals delayed one quarterperiod, one half period and three quarter periods therefrom forcontrolling said square wave signal generating means.

ag UNITED STATES PATENT OFFICE CR'HHCATE OF CECTION Pa N 3, 778,601 a dDecember 11, 1973 Inventor(s) Larry D. Brock and John E Games It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 3, line 25, change "equal inverted to" to inverted to equalColumn 3, line 42, change "an" to on Column 3, line 67, change"illustrates Fig. 3 illustrates" to However, Fig. 3 illustrates Signed dsealed this 25th day of June 197M.

(SEAL) Attestz MQFLETGEER JR, C o MARSHALL DANN Attesting OfficerCommissioner of Patents 533 UNITED STATES PATENT OFFICE I g 7CERTIFICATE OF CORRECTION Patent No. 3,778,601 Dated December 11, 1973Inventor(s) Larry D. Brock and John E. Games ppears in theabove-identified patent It is certified that error a hereby corrected asshown below:

and that said Letters Patent are Column 3, line 25, change "equalinverted to" to inverted to equal Column 3, line 42, change "an" to onColumn 3, line 67, change "illustrates Fig.1 3 illustrates" to However,Fig. 3 illustrates Signed and sealed this 25th day of June 19714.,

(SEAL) Atteat:

EDWARD MJLETCHERJ'R. I c. MARSHALL DANN Atteeting Officer v Commissionerof Patents

1. A navigation computer comprising: means providing a first carriersignal X sin wt and a second carrier signal Y sin wt, the magnitudes ofwhich are representative of X and Y coordinates of an aircraft withrespect to a selected way point; storage means presenting a selected oneof a plurality of designated track angles in the form of a multibitgroup of digital signals; a counter, the output of which isrepresentative of track angles, said counter being advanced from a zerocount to a count relating to maximum track angle during each cyclicperiod; means comparing the output of said storage device with theoutput of said counter and for generating a signal when the outputsthereof are equal, and means responsive to said compare output signalfor generating a first square wave signal at the same frequency as saidcarrier signal but delayed in phase from sin wt by a designated trackangle phi corresponding to said selected way point, and a second squarewave signal at the same frequency as said carrier signals but delayedfrom cos wt by said angle phi ; means for combining said carrier signalswith said first square wave signal to generate cross track error as Xcos phi - Y sin phi ; and means for combining said carrier signals withsaid second square wave signal to generate along-track distance as X sinphi + Y cos phi .
 2. An aircraft navigation computer according to claim1 wherein said square wave signal generating means includes furthermeans responsive to said compare output signal for generating signalsdelayed one quArter period, one half period and three quarter periodstherefrom for controlling said square wave signal generating means.
 3. Anavigation computer comprising: means providing a pair of carriersignals the magnitudes of which are representative of orthogonalcoordinates of an aircraft with respect to a selected way point; storagemeans presenting a selected one of a plurality of designated trackangles in the form of a multibit group of digital signals; a counter,the output of which is representative of track angles, said counterbeing advanced from a zero count to a count relating to maximum trackangle during each cyclic period; means comparing the output of saidstorage device with the output of said counter and for generating asignal when the outputs thereof are equal; means responsive to saidcompare output signal for providing square wave signals at the samefrequency as said carrier signals, one of said square wave signalsdelayed in phase from the sine of said carrier by a designated trackangle corresponding to said selected way point, and the other of saidsquare wave signals delayed from the cosine of said carrier signals bysaid designated track angle; four synchronous demodulators, a first pairthereof synchronously demodulating respective ones of said coordinatecarrier signals in response to one of said square wave signals, theother pair thereof synchronously demodulating respective ones of saidcoordinate carrier signals in response to the other of said square wavesignals; and means for summing the output of each synchronousdemodulator with the output of the other synchronous demodulator of thesame pair, the outputs of said summing means comprising cross trackerror and along track distance to the way point.
 4. An aircraftnavigation computer according to claim 3 wherein said square wave signalgenerating means includes further means responsive to said compareoutput signal for generating signals delayed one quarter period, onehalf period and three quarter periods therefrom for controlling saidsquare wave signal generating means.